First, in Photos put the photos you want to use for your screensaver into an Album. In iTunes File/HomeSharing, select "Choose Photos To Share With Apple TV ...". In the window that opens, at the top check "Share Photos from:", choose Photos from the dropdown menu and click the "Selected albums" radio button. Now in the middle of the window in Albums, check the album you just created which has the photos you want for a screen saver on your Apple TV. Click Done, and apply the changes.
Now on your Apple TV, open Settings/General/Screensaver/Photos. In the Choose Photos window, under Computers choose the Photos library which you just shared. In the library, choose the album with screensaver photos which you created.
That should do it.
Alternatively, if you have iCloud you could create a shared album, then choose it in the Choose Photos window.